Disc drives are used as primary data storage devices in modern computers and computer networks. A typical disc drive includes a head-disc assembly (HDA) housing one or more magnetic discs which are rotated by a spindle motor at a constant high speed and accessed by an array of read/write heads which store data on tracks defined on the disc surfaces. Electronics used to control the operation of the HDA are provided on a printed wiring assembly ("circuit board") which is mounted to the underside of the HDA.
Each head is typically provided with separate read and write elements, with a common configuration utilizing a thin film, inductive write element and a magneto-resistive (MR) read element. Data are written by passing a write current through the write element, with the write current generating a time-varying magnetic field which selectively magnetizes the disc surface. Previously written data are read using the read element to transduce the selective magnetization of the disc to generate a readback signal which is used by a read channel to reconstruct the data. An interface circuit buffers and controls the transfer of data between the disc and a host computer.
Technological advancements in the art have resulted in continued improvements in disc drive data storage capacities and transfer rates. It has not been at all uncommon for each successive generation of drives to provide substantially twice the data storage capacity as the previous generation, at an equal or improved data transfer rate. Design cycle times are also being shrunk to the point that a new generation of drives is typically introduced into the marketplace every few months.
The commercial success of disc drives is not only a result of the costeffective manner in which vast amounts of user data can be stored and retrieved, but also in the demonstrated reliability of the typical disc drive over a relatively long operational life. Nevertheless, for applications where data integrity is critical, methodologies have been developed to further enhance the ability of disc drives to consistently and accurately store and retrieve data.
One such methodology is the grouping of a plurality of drives into a multi-drive array, sometimes referred to as a RAID ("Redundant Array of Inexpensive Discs"). Since their introduction, RAIDs have found widespread use in a variety of applications requiring significant levels of data transfer, capacity and integrity performance. Various RAID architectures employ mirroring (simultaneously writing data to two or more identical drives), striping (writing portions of the data across multiple drives) and interleaving (employing various types of error detection and correction schemes at multiple levels to ensure data integrity).
Another particularly useful methodology to maximize data integrity is through the use of write verification, which involves the writing of data to a disc followed by a subsequent read operation where the previously stored data are retrieved from the disc to ensure the data were correctly written. However, such write verification operations undesirably decrease the data transfer performance of the disc drive, as each write operation requires each sector to which data are written to be accessed at least twice: first, when the data are written, and second, when the data are subsequently read back for verification. Conventional write verification techniques accordingly impose a severe penalty on disc drive performance, limiting data transfer rates to levels substantially below that which would be otherwise achievable.
As consumer demands continue to drive further advances in data transfer rate and integrity performance, there remains a continual need for improvements in the disc drive art whereby these often mutually exclusive characteristics can be optimized. It is to such improvements that the present invention is directed.